Optical illusion shows you can’t always believe what your eyes perceive

This particular optical illusion has been around for awhile, as it was first detailed by psychologist Tom Cornsweet in the '60s, but since it's making the rounds on the internet right now, it's worth it to take a deeper look. Overall, it's a fairly simple illusion, but it's a very powerful one.

In the image to the right, at first glance, it looks like the two tiles are three-dimensional and angled away from us. This happens, even with a two-dimensional static image, because of the visual cues — the shading of the tile edges, the perspective of the landscape, and the shadow on the 'ground'.

So, perceiving it as a three-dimensional object, with the light source above, our brain automatically applies what we've learned in the past about how light and shadow affect colours. It assumes that if the light source was removed or it illuminated both faces equally, that the top face would be a dark shade of grey, and the bottom face would be a light shade of grey (or even bright white).

Putting a finger up in front of the image to block out the middle part (as they suggest in the video) definitely gives a sense that the shades are the same.

However, just to show it a bit better, to the right is the same image, but with one change: the left side of it is flipped upside-down. The edges between the tiles are different, obviously, but as you can see, the faces of the tiles blend together perfectly.

They are, indeed, the exact same shade.

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Another, similar, illusion that's being paired with this one on various sites is the checker shadow illusion (check it out here and a great video of it here). I swear, no matter how many times the girl in the video moves the centre tile to the edge and back, it still looks lighter when it's in the centre! How does this one work? The four 'darker' tiles around the centre tile are actually the exact same shade as the centre tile, but the shadow from the cylinder just makes them look darker. Why does the centre tile look lighter? It's completely in the shadow, whereas the tiles surrounding it are only partly in the shadow. In the same way as the Cornsweet illusion above, the contrast of the lighter and darker regions of the shadow makes our brain perceive those tiles as overall darker.

Science teaches us to always question what we see and there's a very good reason for that, since our own brains sometimes work against us to present things as contrary to reality! It's only by investigating that we can test what we see, and truly know what's really going on. Don't let that stop you from enjoying these illusions though, because they not only make you think, but they're lots of fun too!

(Images courtesy: Dale Purves, R. Beau Lotto and Surajit Nundy)

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